150 



R. T. Prentki et al. 



10.0 



o 



. 1.0 



c 

 o 



u 



•o 

 o 



o 

 E 



0.1 



_ • _ 



"^ • ■ 



^^^^ . • • - 



300 



400 500 600 



Phosphate Sorption Index 



700 



800 



FIGURE 4-29. Pondwater primary productivity (9 August 1973) 

 and phosphate sorption indices for the underlying sediment. The 

 intensively studied IBP ponds (A-B, C, D, E, J and X) are 

 denoted by hollow circles and the other watershed ponds by 

 solid circles. The regression line shown is for the intensively 

 studied ponds only. 



grazing and macrophyte senescence peak, phytoplankton can greatly 

 exceed this lower limit for productivity. 



If additional watershed ponds, especially those with phosphate 

 sorption indices over 625, are included in the regression, the correlation 

 disappears. Those ponds with open centers and small macrophyte 

 populations tend to fall closest to the original regression line. No pond 

 falls much below the line, as is to be expected if the regression line defines 

 a lower limit for phytoplankton productivity. Few details are known of 

 these additional ponds; either higher internal loading of phosphorus or 1-3 

 week earlier fall blooms in the disparate ponds than in the intensively 

 studied ponds would account for their apparently elevated productivity. 



Conclusions 



Only small changes are observed in seasonal pond phosphorus 

 concentrations. This is not so much a result of low rates of phosphorus 

 supply and utilization as it is of strong buffering action by sediment. Partly 

 because of this buffering, phosphorus-sensitive biological indicators such 

 as primary productivity more closely follow seasonal trends in phosphorus 

 supply rates than seasonal trends in concentration. 



Phosphate concentrations in the water column can, to some degree, 

 be related back to inorganic phosphorus levels of the surface sediment. 



